If you're going to base the boxing part on one fight, then I think the fairest comparison to MMA is Silva/GSP. That is the one fight everyone in MMA wants to see and it's not likely to ever happen.
Silva/GSP is probably the biggest fight that could be made in MMA but it's more fantasy than reality. They both fight in separate divisions where Pacquiao and Mayweather do fight in the same division. Plus it's not like the sport of MMA is dependent on that one fight like Boxing is with there's, there's plenty of other big superfights from MMA people wanna see, in Boxing, not so much. That fight got talked about a lot but mostly when GSP and Anderson had their divisions cleared out now GSP has Condit, Kampmann, Hendricks, and maybe Diaz and Anderson has Sonnen, Munoz, Weidman, Shields, Boestch, and Lombard.